Afte a great deal of slogging through the mud I was able to get a view of the back side of the stump. I crouched down to minimize my threat posture, focused thecamera on the hole, and was rewarded by seeing the grackle exiting the nest hole. I was astonished, but looked it up in Birds of North America and it said "Unusual nest sites include birdhouses, tree cavities (e.g., old woodpecker cavities), buildings (e.g., barns), and occupied nests of Osprey and Great Blue Heron"